Either in a magnetic mass spectrometer or in a time-of-flight mass spectrometer, ions are accelerated by a high voltage and introduced into an ion analyzer for performing a mass analysis. FIGS. 7(a) and 7(b) show a conventional mass spectrometer. FIG. 7(a) illustrates the manner in which a high voltage is applied to the ion source of the instrument. FIG. 7(b) illustrates the potentials at various locations in this instrument as well as the movement of ions. An accelerating voltage Va is applied to the ion source, indicated by 1, from an accelerating voltage source 2. The ions are accelerated by the high voltage, pass through a slit 3, and enter an ion analyzer 4 that is at ground potential. For example, this analyzer 4 is composed of an electric field and a magnetic field. In this analyzer 4, the ions are separated according to mass and detected by a detector 5 at ground potential.
In this instrument, a high voltage is impressed on the sample inlet portion of the ion source and so it is necessary to use an electrically insulative sample inlet device in introducing a sample. Furthermore, the ion source and the sample inlet portion need to be electrically isolated from the other portions. Especially where a gas chromatograph or liquid chromatograph is directly connected to the ion source and components separated by the chromatograph are introduced, it is highly likely that an electric discharge is produced due to a large potential differential across the interface between the ion source and the chromatograph.
Accordingly, a mass spectrometer whose ion source is placed at ground potential has been proposed, as shown in FIG. 8(a). The ion source, indicated by numeral 1, is at ground potential. Other components, i.e., an accelerating voltage source 2, a slit 3, an ion analyzer 4 and a detector 5, are electrically isolated from their surroundings and placed at a high negative potential. In this instrument, the sample inlet portion of the ion source can be placed at ground potential, as shown in FIG. 8(b). Therefore, any special device, such as an insulative sample inlet device, is unnecessary. Furthermore, this instrument has the advantage that connection with other analytical means, such as a gas chromatograph, is facilitated. However, it is necessary to provide a large-scale insulating mechanism, because a large potential difference exists between those components other than the ion source and ground potential. In addition, any countermeasure must be taken to prevent the human operator from getting an electric shock.